The long-term goal of this research is to understand the principles underlying patterning of cell types in the plant epidermis, and in particular the function of stem cells in development. Because plant cells do not migrate, patterns of cell division and position-based mechanisms of cell fate choice are critical to tissue patterning. Stomata are used as a model system because they present a unique opportunity to examine how stem cell maintenance, asymmetric division, cell fate choice, and positional signaling contribute to the tissue-building process in determinant organs. Stomata are conspicuous epidermal structures composed of two guard cells that function as turgor-operated valves to control gas exchange and water loss. In Arabidopsis, they are spaced non-randomly with a minimum of one intervening epidermal cell between each. This non- random spacing is accomplished by strict control of both the spatial location and polarity of asymmetric divisions forming new precursors that arise between pre- existing stomata as the leaf grows. Correct execution of patterning divisions and regulated precursor behavior requires cell-cell communication of positional information through a signaling pathway that includes both leucine-rich repeat receptor kinases (LRR-RLKs) of the ERECTA family, the LRR receptor-like protein TOO MANY MOUTHS (TMM), the subtilisin-like processing protease STOMATAL DENSITY AND DISTRIBUTION 1 (SDD1) a MAPKK Kinase cascade (YODA, etc). There is currently a large gap in understanding the molecular genetic control of precursor (stem cell) fate in the stomatal cell lineage and epidermal patterning. Here we propose to characterize two genes that were identified by global expression profiling of tmm mutant and wild-type tissues. The specific aims of this project are to investigate the roles of these genes using loss- of-function and ectopic expression phenotypes, patterns of expression, and genetic interactions with other loci controlling stomatal biogenesis. Ultimately, we aim to build a framework to better understand how signaling from surrounding cells maintains or inhibits stem cell fate. The proposed research may provide insight into the molecular characteristics of both plant and animal stem cells, as well as how stem cells are maintained in an indeterminate state by positional signals emanating from the local environment ( niche signals ). Many human diseases (Crohn's disease, polycystic kidney disease, hypothyroidism, etc.) as well as clinical challenges (e.g. neural regeneration after traumatic injury or stroke, inflammation, apoptosis and cancer biogenesis, etc.) are directly caused or impacted by the function of LRR- receptors. A better understanding of how LRR-receptors function in signaling pathways may provide new perspectives on the function of this class of medically-relevant proteins. [unreadable] [unreadable] [unreadable]